In heating substances of various types, it is necessary to control the heat generating element to prevent an excessive rise in temperature. This control may be by way of an interval timer which activates the heating element for a predetermined period. Alternatively, the heating element may be controlled by a temperature responsive unit which reacts to the temperature of the load environment such as the air within an oven or the heating plate on an appliance.
The use of a sensing element responsive to the temperature of a heating plate to control the heat input to an appliance is taught in U.S. Pat. No. 3,328,561. That patent describes a ferrite element which is normally attracted by a permanent magnet except when the ferrite is heated to a temperature exceeding its "Curie Point". In this device, at the control point, the temperature of the ferrite must necessarily be above the temperature of the heating plate. A ferrite element would, therefore, appear to be unsuitable for use to control a microwave oven since the electromagnetic energy heats the load without producing a corresponding increase in temperature in the air in the oven. Electromagnetic ovens do not get hot the way conventional ovens do and they cannot be controlled by a conventional thermostat. According to the usual practice, a microwave oven is controlled by a timer in which the size of the load, its character, water content, initial temperature and the like are considered and, based on experience, an appropriate setting is selected. This, however, requires great skill or experience to achieve dependably satisfactory heating. There remains, therefore, the need for a means to control a microwave oven which is responsive to the condition of the load in the oven.